This invention relates to an electronic ballast for a fluorescent lamp, and in particular to such a ballast that is capable of operating with a range of fluorescent lamps of different power ratings.
Fluorescent lamps are driven by an electronic ballast. Conventional ballasts are, however, normally designed for use with a fluorescent lamp of a particular power rating (which may in a commercial or industrial fluorescent lamp typically be in the range of 18 W to 70 W) and ballast manufacturers usually offer a range of ballasts adapted for use with differently powered lamps.
A conventional form of electronic ballast is the half-bridge inverter type shown in FIG. 1. This form of ballast comprises a resonant tank formed by a resonant inductor Lr and a resonant capacitor Cr. The resonant capacitor is connected across the lamp and the two switches S1 and S2 are switched on and off alternately at high frequency (typically between 20 kHz and 70 kHz) so that a high frequency ac voltage Vxy can be created across the resonant circuit. Before the lamp""s arc is struck, the lamp impedance is very high (and is almost an open circuit). The voltage across the capacitor can be very high (and resonance and this high capacitor voltage can cause the arc of the lamp to strike. Once the lamp is on, the impedance drops and the resonant inductor functions to limit the current flowing through the lamp.
By controlling the switching frequency of S1 and S2, the frequency of Vxy, which is also the operating frequency of the lamp, can be controlled. In particular, by increasing the switching frequency the impedance of the resonant inductor is also increased and this functions to reduce the power of the lamp.
According to the present invention there is provided an electronic ballast system capable of driving a fluorescent lamp within a range of nominal power ratings, comprising: means for varying the switching frequency of an electronic ballast over a range including at the bottom end of the range the switching frequency necessary for a lamp at the upper limit of said range of power ratings.
By means of this arrangement, a single electronic ballast may be provided that is capable of driving any fluorescent lamp within a range of nominal power ratings.
In a preferred embodiment the electronic ballast system comprises means for sensing the power rating of the lamp, and means for selecting a corresponding switching frequency. Preferably the sensing means comprises means for sensing the peak lamp voltage.
In a preferred embodiment the means for varying the switching frequency of said ballast comprises a voltage controlled oscillator, and the means for selecting a corresponding switching frequency comprises means for generating an input signal to said voltage controlled oscillator. The means for generating an input signal may comprise means for providing a compensated inverted peak lamp voltage and means for mapping the compensated peak lamp voltage to an input signal for said voltage controlled oscillator.
In a particularly preferred arrangement the electronic ballast system may comprise initial ignition control means that controls the switching frequency of the ballast to provide an arc-striking switching frequency selected such that it will ignite any lamp within the range of nominal power ratings. Preferably, also, means are provided to supply to said lamp a warm-up current before said lamp is ignited.
Another preferred aspect of the present invention is that it may comprise means for sensing lamp failure, and this means may comprise means for detecting when the peak voltage of said lamp remains high for a continuous period.
Viewed from another broad aspect the present invention provides a method of driving a fluorescent lamp within a range of nominal power ratings, comprising: (a) providing an electronic ballast capable of operating at a range of switching frequencies including at the bottom end of the range the switching frequency necessary for a lamp at the upper limit of said power range, (b) identifying the nominal power rating of a said lamp to be driven, and (c) selecting an appropriate switching frequency for the said lamp.